The x11 FPGA were kept secret, but the hashrate showed they must have existed since it was unprofitable for long periods of time to mine DASH but it had massive amounts of hash power.
Keep in mind that a lot of major farms pay less than 3c/kwh for electric - back in the Darkcoin days even with 8c I was break-even at WORST (but summer rate at 14 convinced me to move to something else, and I never looked back).
No need for hypothetical FPGA-based X11 miners to explain why it got unprofitable for many small miners.
I doubt that ETH going to PoS will affect it's price as such - it WILL kill the ability of miners to mine it once it goes 100% PoS, but miner profitability does NOT drive price - price drives profitability.
No there were definitely x11 miners, there were single nicehash addresses with 30+ GH mining which in GPU days was pretty massive. the reason it was unprofitable was because of the difficulty not so much the coin price. The hashrate kept growing despite the price not growing and other coins being more profitable. it was a bear market for all coins but people would rotate to whatever was turning a profit, yet the hashpower being thrown at x11 always went up even if the coin price dropped. FPGA aren't hard, they sell kits for them. I'm not smart enough to do it but it's not rocket science. Anyway nobody was publicly selling them except for the scam cloud mining guy so no hard proof but there were people on bitcointalk claiming to have seen x11 fpga farms in China.
Only two other coins have ASIC, Bitcoin and Litecoin and both of them had FPGA before ASIC so it would actually be odd if X11 didn't have FPGA. The kits are cheap and design is easier than ASIC. It would be pretty odd if nobody did FPGA before jumping to ASIC.
One reason FPGA aren't sold publicly is they are easy to copy, so all the work put into design if somebody got their hands on it they could buy a kit and reproduce it easily. I'm not an FPGA expert, just a forum lurker but from what I read there are FPGA that can protect from copying but they are expensive. Those with the skills to program the FPGA make more mining for themselves building up a farm rather than selling a couple and having their deisgn copied and then they have to compete with others, so it makes economic sense to keep FPGA secret.
Decred algo was a good algo for FPGA, if the devs were smart they designed some FPGA before releasing the coin so they could mine super efficient. ANd actually I remember Vanilla coin or something similiar to that the dev actually did build FPGA and had videos of it. I wasn't into that coin so not sure if he sold or shared his design or what but I remember seeing it and it wasn't so fast it made GPU unprofitable but it was super efficient using very little power compared to GPU.
Anyway they are more common than most think, and if a coin made it all the way to ASIC I'd be willing to bet it had an FPGA along the line.